Ion source



May 16; 11950 L P, MITH 2,507,652

ION SOURCE Filed 001;. 4, 1940 3 Sheets-Sheet 1 3000 V. QC. 600 V. DC.

no'v. 0.0

IN VENTOR.

ATTORNEY.

Vacuum Pam 21s and Gauges L. P. SMITH May 16, 1950 ION SOURCE 5Sheets-Sheet 2 Filed Oct. 4, 1940 [qua (r055 5mm W H 0 mm m W5 m i M e ZW L Y B m W M W rm W fi 6 May 16, 19.50

Filed Oct. 4, 1940 L. P. SMITH ION SOURCE Kazan/z am 5 and Gauyai 5Sheets-Sheet 3 INVENTOR.

ATTORNEY.

Patented May 16, 1950 ION SOURCE Lloyd P. Smith, Ithaca, N. Y., assignorto Cornell Research Foundation, Inc.', Ithaca, N. Y., a core por'ation'of New York Application October 4, 1940, Serial No. 359,742

3 Claims. (01. 250-419) This invention relates to matter andelectricity, and particularly to the production of an ion source ofrelatively high efliciency, so that the atoms of various materials maybe more readily iQniZed. Ionization is useful in the chemical andphysical fields, as is well known- -for example in chemicalcombinations, production of isotopes, nuclear transmutations, productionof light, transfers of energy an d various ion sources have beendeveloped in the past. The principal object of the present invention isto improve the eff ciency of ionization and reduce the cost andcomplexity of such apparatus, with a view to furthering the practicalutilization of such phenomena. Various other objects will becomeapparent as the description proceeds.

One general method of ionization has been to direct a beam of electronsthru volatilized matter, so that the atoms in the vapor are ionized byelectron collisions. As the spaces in the atomic particles arerelatively great, and the electrons are relatively small and travel withgreat speed, the collisions have not been very frequent. In order toincrease the probability of collision, I have invented a method by whichthe electrons are compelled to pass back and forth thru the atoms agreat many times, until the collision takes place, instead of merelypassing thru once. Another important feature of the device is that allof the ions thus produced can be extracted for useful purposes withoutthe loss by escape heretofore common. The efficiency of ionization isthereby greatly increased. The ions produced by such collisions arecontrolled so that they pass down the axis of the electron beam, inwhich they are held by the increasing electron space charge as theelectrons slow down to reverse their direction. In general the inventionis characterized by oscillating electrons with the ion path controlledby the electron beam.

Referring now to the drawings, Fig. 1 is a schematic diagram of one formof the apparatus, of which Fig. 2 shows a typical constructional form ingreater detail, in cross-section. Fig. 3 is a perspective view of onetype of cathode. Fig. 4 is a perspective view of another type ofcathode. Fig. 5 is an elevation view of the water cooled first plate.Fig. 6 is a perspective view of the fourth, fifth and sixth platescomprising the ion gun.

Fig. '7 is a cross-sectional view of another typical form in which acontinuous stream of atoms is intercepted by a transverse electronstream. Fig. 8 is a diagram of the potential fields. Fig. 9 is across-section on the plane 99 of Fig. 8. Fig. 10 is a cross-section onthe plane IlL-lll of Fig. 8. Similar reference numerals refer to similarparts thruout the various views.

The apparatus may be described in general as consisting of a cathode foremitting electrons, a positive field toacceleratethe electrons up to thepoint where the stream of atoms to be ionized is. crossed, and, then anegative field on the other. side of said stream to stop and reverse theelectrons that have slipped thru and compel them to pass thru the streamof atoms repeatedly until an electron strikes an atom and ionizes it; inwhich case the ionized atom escapes along the axis of the electron beam,in which it is held by the electronic space charge. The active electronsoscillate with great rapidity back and forth between the cathode whichrepels them on the one hand and the negative field on the other side ofthe atomic stream which repels them on the other hand; so that theelectrons are bounced back and forth indefinitely until they finallycollide with atoms. It will be seen that the chances of electronscolliding with atoms to ionize them are thus very greatly increased; andthis has been confirmed by the results obtained.

In the apparatus illustrated in Fig. l and Fig. 2 for example, the fieldwhich compels the electrons to shuttle back and forth thru the atomicstream is produced by a number of suitably charged plates, of whichvarious numbers may be used, six being shown in the example illustrated.Of these, plate I, plate 2 and plate 3 are positively charged toaccelerate the electrons, and plate 4, plate 5 and plate 6 arenegatively charged to retard, reverse and accelerate the electrons backagain in the opposite direction. The electrons are originally shot offby the cathode l, adjacent plate I, and the ionized atoms are caught bythe collector 8 on the opposite side beyond plate 6. Holes 35 in thecenters of the plates provide a path thru which the electron beamextends, andthe atoms to be ionized are introduced at any suitableintermediate pointwhere they will cross the path of the electrons, asfor example between the plates 2 and 3. The electrons are held to-.gether by a uniform magnetic field parallel to the axis of the electronbeam, this field being created by the magnetizing coils Ill.

The foregoing parts, shown in the schematic view, Fig. 1, are shown inthe greater detail of an actual structure in Fig. 2, in which the platesare enclosed in a Pyrex glass T-shaped flanged pipe fitting l2, onebranch of which carries the lead-in plate l-3 supporting the variousplates described, the opposite branch containing the ion collector 8 andobservation window l5, and the. third leg of the T leading to the vacuumpumps I! by which the low pressure in the apparatus is maintained.

The leads to the cathode and the various plates described extend thruthe lead-in plate l3, and these leads are also used as a convenientmeans for supporting the cathode and plates in the glass T member l2.The lead-in plate 13 is clamped over the opening of the glass T memberI2 by the bolts 20, using a rubber packing 2| to ried on the end of thewires 23 and 24, and while this particular form of the cathode isimmatebeing relatively small and fast particles as compared with theatoms, and the spaces between the atoms being relatively great, it isknown from past experience that an electron may shoot thru a great manyatoms without collision. Therefore, in order to increase theeffectiveness of the r apparatus, the present invention compels suchrial, so long a it is a metal heated sufficiently to give off electrons,the cathode 1 illustrated is made of a strip of roughened or protrudednickel oxide coated on both sides. This is backed by a heat shield 28and the general form may be seen in the perspective view of Fig. 3.

The first plate I which carries a positive electric charge is cooled bythe water pipe 30, which is soldered or welded to it and which alsoserves as the electrical connection and supporting member. This plate 1is shown in elevation in Fig. as well as in cross-section in Fig. 2, andit will be seen from Fig. 5 that it has five holes 2, 3, 4, 5' and 6around its rim as well as a notch 3| in the top. The five holes are forthe passage of the leads to the plates 2, 3, 4, 5 and 6 respectively,which leads also serve as supports. For the same reason plate 2 has foursuch holes and plate 3 has three. Bringing the leads thru the plates inthis manner gives rigidity to the system, Plates 4, 5, and 6 comprise aseparate unit (also shown in Fig. 6), which will be referred to as theion gun, and this is mounted rigidly and accurately parallel by means ofthe glass beads 33, which also serve as insulation. At points betweenthe glass beads 33 the plates 4, 5, and S are provided with projections34 for electrical connections and mechanical supports. In operationthese plates t, 5, and 6 are negatively charged so as to repel theelectrons. All the plates 1 2, 3, 4, 5, and 6 are provided with centralholes 35 to permit the passage of the electron beam. 7

A cylindrical glass wall 36 is provided between plates 2 and 3 toenclose a chamber 31 into which the gas or vapor containing the atoms tobe ionized may be introduced. These gaseous atoms are led in thru thetube 40 which extends over the top of plate I thru the notch 3| shown inFig. 5. This sufiices when the material to be ionized is in the form ofa gas or vapor brought in from an exterior source. The material to beionized can also be volatilized within the apparatus, as

is sometimes desirable with solid materials for example, and that willbe described further in connection with Fig. 7.

The apparatus is energized as shown for example in Fig. 1, the plates i,2, and 3 being main tained at the same positive potential and plates 4,5, and 6 diminishing in steps to the negative potential of the collector8. The power for the cathode '1 and the first three plates I, 2, and 3was in a typical installation, supplied by a 600 volt D, 0. generatorsystem; the power for the plates l, 5, and G and collector S was from a3000 volt D. C. system; and the magnetic coils l0 were energized by a110 volt D. 0. system 52. Any suitable sources of power may of course beused, and those shown are merely illustrative.

In operation the electrons emitted by the cathode 'I are accelerated bythe plates I, 2 and 3, due to their positive potential, the electronstream passing thru the holes 35 in the plates and shooting thru theatoms of the material to be ionized, as that vapor fiows down betweenplates 2 and 3 and crosses or flows thru the stream of electrons. Thepurpose of the apparatus is to cause collisions between the atoms andelectrons and thereby create the ionized atoms desired. The electronselectrons-that have passed thru the atoms without collisionto return andpass thru the stream of atoms repeatedly until collision results. Toaccomplish this the plates 4, 5, and 6, known as the ion gun, are givensuch reversed potentials as to slow down, stop and turn back theelectrons that are still free and have not collided with atoms. Thesereturning electrons are accelerated by the potentials of the plates 4,5, and 6 back toward plate 3; and passing thru the region of plates 3,2, and I, encounter the negative field of the cathode 1, which togetherwith the positive plates l, 2 and 3 sends them back again thru the atomsas before. This shuttling process is repeated indefinitely, and theelectrons oscillate back and forth until they are picked up by one ofthe plates or stopped by collision with atoms. In the latter event theionized atoms emerge along the axis of the electron beam, being heldthere by the electronic space charge.

The controlling potentials created by the electron and ion space chargesin the magnetic field are illustrated in Fig. 8, Fig. 9 and Fig. 10. Themagnetic field created by the coils I0 restricts the electrons to abeam, and the resulting electron space charge, which increases as theelectrons slow down and come closer together, forces the ions to travelalong the axis of the beam. The distribution of potentials is showndiagrammatically in Fig. 8. The net eiTect of the electron and ion spacecharge is to create a saddle point in the potential distribution betweenplates 2 and 3, as indicated in the upper part of Fig. 8, so that allthe ions are confined to the beam and all those produced on the ioncollector side of the saddle point are accelerated toward the collector8. The potential also varies as one passes farther from the axis of thebeam, so that cross-sections of the saddle such as Fig. 9 and Fig. 10are U- shaped. It will be noted that the magnetic field does not controlthe ions directly; but that the magnetic field controls the electrons,and the electrons control the direction of the ions. The fact that theelectrons (which are negative particles), increase their space charge asthey slow down, results in confining the positive ions to a beam whichpermits the ions to be readily extracted along the axis. Thisarrangement of the space charge to confine and extract the ion beam isanother characteristic feature of the present invention.

Considering the operation in greater detail, it the electrons were notstopped until they were almost at the surface of the collector, the ionswould be held in the beam by the negative space charge and all the ionswould reach the collector. If the potentials on the ion gun were suchthat the electrons were stopped sooner, say in the region of plate 4,the ions would tend to diverge from that point. It is possible in thelatter case to adjust the potentials on the plates to produce a radialfield which will just balance out the radial field due to space chargeand keep the ions moving in parallel paths.

The extent of the electron beam can easily be seen by the excitationproduced in the hydrogen. The manner in which the point where theelectrons are stopped draws away from the collector toward the thirdplate, as the potentials on the ion gun and collector are made morenegative, is readily observed. During this process the ion current firstdecreases due to the fact that, when the collector potential is severalhundred volts below the cathode potential, the electrons are stopped aconsiderable distance from the collector and the ions diverge becausethe accelerating voltage is not sufficient to collimate them. Withcontinued decrease in the collector potential the ion current passesthru a minimum and then climbs back to its original value, when thepotentials reach the values theoretically required.

The fact that this was experimentally found to be true, indicates notonly the correctness of the above explanation but also that the gaspressure was much higher where the gas was introduced and that only asmall fraction of the ionization could have taken place between plate 3and the collector. As would be expected, varying the potentials of theion gun plates makes considerable difference in the ion current when thecollector potential is far below that of the cathode,

but almost no difference when the collector and cathode are at almostthe same potential In some cases,especially when dealing with a solidmaterial to be ionized, it is desirable to vaporize it inside theapparatus, instead of introducing the gas or vapor from an exteriorsource. The form shown in Fig. 7 provides means for vaporizing thematerial inside the apparatus, but otherwise is substantially the sameas Fig. 2. Referring now to Fig. 7, the material to be ionized is placedin the heater or furnace 60, which is covered by a double heatinsulatinglid iii. The furnace 60 contains a heating coil 62, preferably oftungsten or similar high temperature material, which receives itscurrent thru the lead 63. The bottom of the furnace 50 is provided withorifices 65 and 66 thru which the stream of vaporized materialcontaining the atoms to be ionized is directed down across the electronbeam emitted from the cathode 1', (corresponding to the cathode 1 ofFig. 1 and Fig. 2). The furnace 60 and its associated parts are mountedon a tubular member 68 corresponding to the plates 2 and 3 of Fig. 2,and cooled by water tubes 69 (corresponding to the tubes 30 of Fig. 2)which also serve as supports. In the bottom of the member 58 is locatedthe receptacle to catch the residue.

The cathode 1 shown in Fig. 7 differs somewhat from the one shown inFig. 2, and consists of a casing 1! containing a molybdenum stocking 72filled with an activating salt, and a number of vanes 13 (shown incross-section) to increase the emitting surface. This is sometimes knownas the Hull type. The particular form of cathode is immaterial, thetypes shown being merely by way of example.

The operation is in general the same as Fig. 2, the stream of vaporizedmaterial passing down from the tube 66 across the line of the openings35 containing the electron beam in which the electrons oscillate rapidlyback and forth, until by chance they strike atoms to form ions, which isthe desired purpose. The resulting ions escape down the axis of themagnetic field created by the coils ID, for the reason that the magneticfield confines and controls the electrons, which in turn confine andcontrol the ions. This is due to the fact that the electron spacecharge, which increases as the electrons slow down and consequently comecloser together, forces the ions to travel along the axis.

It will be seen from the foregoing that the principal features of theinvention are the use of oscillating electrons crossing the atomicstream repeatedly, and the escape of the resulting ions being restrictedto a central path by the space charges in the electron beam.

While I have in the foregoing described certain particular embodimentsof the invention, it will be understood that they are merely forpurposes of illustration to make clear the principles thereof, and thatthe invention is not limited to the particular forms described, but issubject to various modifications and adaptations in diiferentinstallations will be apparent to those skilled in the art withoutdeparting from the scope of the invention as stated in the followingclaims.

I claim:

1. In an ion source operating in a vacuum, the combination of anelectron source providing a beam of electrons, positive potential meansfor accelerating the electrons, negative potential means fordecelerating and reversing the electrons so that they oscillate in theirbeam, means for continuously supplying atoms to the oscillating portionof the electron beam, whereby the atoms may be ionized by collisionswith the electrons, and means for producing a magnetic field to confinethe electron beam and produce an axial path for the ions, and acollector for said ions, the vacuum being held at a moderate value topreclude diffusion of the ions to the surrounding walls.

2. In an ion source operating in a vacuum, the combination of anelectron beam, a magnetic field restricting said beam, means forcontinuously introducing atoms to be ionized into said beam, andnegative electric potential means for reversing the travel of theelectrons in said beam, whereby the electrons may traverse repeatedlythe region into which the atoms are introduced, to increase the chancesof ionization, and a collector for extracting the ions, the vacuum beingheld at a moderate value to preclude diffusion of the ions to thesurrounding walls.

3. In an ion source operating in a vacuum, the combination of a cathodefor emitting electrons, positive potential means for accelerating saidelectrons, means for continuously introducing atoms in the region ofsaid positive potential means, whereby the atoms may be ionized,negative potential means for decelerating the electrons to increasetheir space charge, so as to confine the ions to the axis of theelectron beam, l8, magnetic field restricting the electron beam, andmeans for extracting the ions, the vacuum being held at a moderate valueto preclude diffusion of the ions to the surrounding walls.

LLOYD P. SMITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,408,053 Wensly Feb. 28, 19222,034,571 Found Mar. 17, 1936 2,081,429 Gaede May 25, 1937 2,219,033Kuhn et al Oct. 22, 1940 2,272,374 Kallmann et a1. Feb. 10, 1942 OTHERREFERENCES Thomson: Conduction of Electricity through Gases, vol. II,pages 48 and 49, published 1933 by Cambridge University Press.

